1. Field of the Invention
The present invention relates generally to methods and apparatus for post-tensioning tendons within concrete structures. More particularly, the present invention relates to methods and apparatus for providing a visual indication of the tension applied to a tendon within a post-tension system.
2. Description of Related Art
Prestressed concrete is a type of reinforced concrete that has been subjected to an external compressive force prior to the application of loads. The compressive force is typically provided by steel tendons within the concrete that are initially tensioned with hydraulic jacks and held in tension by end anchors.
Prestressed concrete is categorized as either pre-tensioned or post-tensioned. Pre-tensioning refers to the method of first stressing tendons and then casting concrete around the prestressed tendons. The concrete cures before releasing the prestressed tendons and transferring the stress from the tendons to the concrete. Post-tensioning refers to the method of casting concrete around unstressed tendons and then stressing the tendons after the concrete has reached a specified strength.
Although the modem method of prestressing concrete may be traced to the late 1920""s, its general use in the United States did not begin until the late 1940""s or early 1950""s. General acceptance and the primary increase in use occurred primarily between 1965 and 1975. Application of prestressing was being made in all aspects of construction including buildings, towers, floating terminals, ocean structures, ships, storage tanks, nuclear containment vessels, bridge piers, bridge decks, foundations, soil anchors, and virtually all other types of installations where normal reinforced concrete was unacceptable. Thus, prestressed concrete and methods for its initial installation for diverse applications is now well known.
After years of service, however, problems associated with prestressed concrete structures became apparent. These problems primarily relate to the premature deterioration of the prestressed concrete structures and the subsequent task of identifying and repairing the deteriorated structures before they experience complete loss of structural integrity.
One of the major problems associated with the failure of such prestressed concrete structures lies in the application of inadequate tensioning forces to the tendon associated with the concrete structure. Typically, at the work site, the workers will utilize the hydraulic jack so as to apply a sufficient tension to the tendon. However, when unsupervised, an inadequate tension can be applied to the tendon. In other circumstances, the tendon will remain completely unstressed. As a result, the ultimate structural integrity of the entire concrete structure can be compromised. After the concrete has set and cured, it becomes very difficult to adequately inspect as to whether the tendons have been properly tensioned.
A problem that is continually faced by the prestressing industry is the ability to provide assurance that the tendons have been properly stressed. Under certain circumstances, spray paint has been applied to the tendon to assure that a tensioning operation has actually occurred. If no tensioning has occurred, then the spray paint will not show the elongation of the tendon as a result of the tensioning. However, the application of spray paint does not provide assurance that the adequate tensioning has occurred. Under other circumstances, an instrument can be applied to the tensioned tendon in order to determine whether the tendon has been properly tensioned. However, this becomes problematic after the concrete has been actually poured and the tendon is concealed within the concrete. If a failure occurs, then it would be necessary to destructively test the concrete and the tendons in order to determine whether the tendons have been properly tensioned. At present, no method exists whereby the supervisor or inspector at the work site can easily inspect the anchors so as to determine visually whether the proper tensioning has occurred. Additionally, and at present, no method exists whereby a determination can be made of the proper tensioning subsequent to the pouring and solidification of the concrete.
In those circumstances where a failure of the concrete has occurred (e.g. a cracked slab or a building foundation), it is often difficult to determine liability or to determine the extent of the inadequate stressing without destroying the remaining concrete foundation. As such, a need developed whereby the tension of the tendon can be inspected subsequent to tensioning so as to determine the cause of the foundation failure.
It is an object of the present invention to provide a method and apparatus whereby the proper tensioning of a tendon can be determined by visual inspection.
It is another object of the present invention to provide a method and apparatus for such visual inspection which is easy to use and easy to implement.
It is still a further object and advantage of the present invention to provide a method and apparatus whereby the inspection of the proper tensioning of the tendon can be carried out without destructive techniques.
It is still a further object of the present invention to provide a method and apparatus which eliminates the need to paint or visually mark the end of the tendon.
It is still another object of the present invention to provide a method and apparatus for facilitating the visual inspection of the tensioning of tendons which is relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.
The present invention is an anchorage for a post-tension system comprising an anchor body having an interior cavity and a generally flat end surface, a tendon extending through the interior cavity of the anchor body, a plurality of wedges in interference-fit relationship between a wall of the interior cavity and a surface of the tendon, a cap member positioned outwardly of the generally flat end surface, and a ring juxtaposed against the inner surface of the cap member. The ring is physically deformable so as to provide a visual indication of the desired amount of tension in the tendon.
In the present invention, the cap member has a diameter no less than the diameter of the ring. The cap has an orifice. The tendon extends through this orifice. The orifice is tapered so as to have an angle of taper matching the tapered interior cavity of the anchor body. The plurality of wedges extend through the orifice of the cap member and into the interior cavity of the anchor body. The plurality of wedges are affixed to a wall of the orifice of the cap member. The ring is received within an annular slot formed on the flat end surface of the anchor body. The annular slot is concentric with the interior cavity of the anchor body.
In the present invention, the ring has an inner diameter greater than the diameter of the interior cavity of the anchor body at the flat end surface. The ring is of a copper material. The ring is initially retained within the annular slot by a light press fit. The annular slot has a section tapering outwardly toward the flat end surface of the anchor body. The desired amount of tension is greater than 28,000 p.s.i. The physical deformation of the ring is crushing into the annular slot.
The present invention is also a method of providing a visual indication of tension applied to a tendon of a post-tension system comprising the steps of: (1) positioning an anchor onto the tendon adjacent an end of the tendon such that the end of the tendon extends outwardly for a desired distance from an end surface of the anchor; (2) placing wedges into an interior cavity of the anchor on an exterior of the tendon; (3) placing a ring around the wedges; (4) placing a cap member around the tendon so as to have an end surface facing a surface of the ring; and (5) tensioning the tendon by applying a compressive pressure against the cap member until the ring member physically deforms. The step of tensioning the tendon includes tensioning the tendon such that the compressive pressure exerted by the cap member is greater than 28,000 p.s.i. upon the ring. The tensioning of the tendon is released such that the wedges will reside in interference-fit relationship between a wall of the interior cavity of the anchor and the exterior of the tendon.